To shout or call out loudly to get someone's attention; also used as an exclamation of greeting or affirmation.
Likely from Dutch 'holla' or similar Germanic shout words, or possibly from 'hallo' (a hunting cry). It entered English as both verb and interjection representing a loud vocalization.
Holla sounds like what it means—the double-L makes your mouth do the same thing it does when you shout—and it's one of those words that's survived for centuries because it just works!
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